In many applications, a communication medium is shared among a number of nodes. The nodes compete with one another for access to the shared communication medium. At any given moment, there may be more than one of the nodes that wish to transmit data over the shared communication medium. A system is typically put in place to facilitate access to the shared communication medium by the various nodes. Various categories of such multiple access systems have been developed.
One category of multiple access systems utilizes contention protocols. Examples of these contention protocols include the ALOHA protocol and the slotted ALOHA protocol, which are known in the art. Here, each node is allowed to freely transmit its data over the shared communication medium at any time or any slotted time. In a system employing a hub, each node sends its transmission to the hub, which then broadcasts the transmission to all nodes. In a system without a hub, each node directly broadcasts its transmission to all nodes. In either case, every node listens to the channel for its own transmission and attempts to receive it. If a node is unsuccessful in receiving its own transmission, the node can assume that its transmission was involved in a collision with another transmission, and the node simply re-transmits its data after waiting a random amount of time. In this manner, collisions are allowed to occur but are resolved by the nodes.
Another category of multiple access systems utilizes carrier sense protocols. Examples include persistent carrier sense multiple access (persistent CSMA) and non-persistent carrier sense multiple access (non-persistent CSMA) protocols, which are known in the art. Generally speaking, these protocols require each node to listen to the shared communication medium before transmitting. Only if the shared communication medium is available is the node allowed to transmit its data. In persistent CSMA, when a node senses that the shared communication medium is not available, the node continually listens to the shared communication medium and attempts to transmit as soon as the medium becomes available. In non-persistent CSMA, when a node senses that the shared communication medium is not available, the node waits an amount of time before attempting to listen to the shared communication channel for an opportunity to transmit. Even though a node listens first before transmitting, there still exists a probability for collisions. When the medium is available, two or more nodes can detect the availability and decide that they are going to transmit data. Various techniques have been developed to handle such collisions.
Yet another category of multiple access systems utilizes contention free protocols. Here, each node can reserve the shared communication medium in order to transmit data. The node can transmit data without colliding with transmissions from other nodes because the shared communication medium is reserved, for a particular time duration for example, for the node's transmission and not for any other transmission. A significant advantage of contention free protocols is that the communication medium is not taken up by unsuccessful transmissions that collide with one another and the resulting re-transmission attempts. This can lead to a more efficient use of the shared communication medium, especially as the number of nodes and number of data transmissions increase.
Some systems utilizing contention free protocols organize the communication medium into a request signal space and a transmission signal space. The request signal space may be organized into a plurality of request segments. A request segment generally refers to a portion of the request signal space that may be used for sending a request. Similarly, the transmission signal space may can be organized into a plurality of scheduled transmission segments. A scheduled transmission segment generally refers to a portion of the transmission signal space that may be used for sending a data transmission. A request to transmit data in the transmission signal space is made in the request signal space. A scheduled transmission segment is assigned to each request. A node transmits data on the transmission signal space after the request for transmission has been granted.
In such a system, request segments may be organized in the request signal space based on time-division multiplexing (TDM), frequency-division multiplexing (FDM), code-division multiplexing (CDM), and/or other multiplexing techniques. For example, when request segments are organized based on CDM, each request segment corresponds to a code channel that can be extracted from the request signal space by applying a particular code word. By utilizing CDM, the request segment is extended in time and spread in frequency. That is, without applying code-division multiple access, the request segment may be confined to a shorter time slot and narrower frequency bandwidth. Thus, the energy of a request can be spread out over a longer time slot and over a wider frequency bandwidth. This allows the transmitter to use less power. Specifically, the arrangement captures the otherwise unused power (power in unoccupied time slots and unoccupied frequency bandwidths) in a low load factor request channel. The use of CDM as applied to request segments can significantly improve request detection performance.
However, systems that organize requests segments based on code-division multiplexing often require precise time synchronization between nodes to ensure proper request detection performance. This can increase the complexity and cost of the system. Thus, to take full advantage of the benefits of a contention free protocol system utilizing CDM request segments, improved methods for reservation of the shared communication medium are needed.